Altitude, don't forget altitude

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I fully concur with this advice.  There are, unfortunately, a number of
environmental variables, many of which are not measured.  For example, i
just completed a 1,000 km round trip through parts of the BC coast
mountains.  On the way to my destination i got 13.1 km/l, while on the
way back it was only 11.5 km/l!  The major differences were air
temperature, routing, vehicle speed and air speed/direction.

The variables that play a role are:
 - elevation,
 - average grades of hills/passes (1,400 m to 1,750 m),
 - ambient temperature,
 - air humidity,
 - average traveling speed,
 - percentage of time spent in which gear (city vs highway),
 - tire inflation,
 - number of passengers,
 - wind speed,
 - type of road surface,
 - state/condition of engine (air/fuel filters, injectors, compression,
coolant temperature),
 - amount of fuel taken at refill (not consistent), and
 - fuel (type/quality).

There are indeed well known standards for "Design of Experiments" that
aim to determine all the variables that will affect fuel consumption.
This is not a trivial undertaking, and requires a fairly large number of
vehicles to take part in order to get a statistically significant
result.  In addition, all measurable variables _must_ be measured all of
the time during the tests.  As a result, these tests are essentially
impractical to carry out by laypeople.

The best way to determine the efficacy of any fuel additive (such as
MMO) is to run the car(s) on a calibrated dynamometer under precisely
controlled conditions on a tank of "standard" petro diesel fuel, then
refuel with exactly the same amount containing the prescribed amount of
MMO.  This is quite feasible, but costs a good deal of money.

Incidentally, i checked out the Walmart store where in live in BC, and
MMO is simply not available.  There were two brands of "injector
cleaner" claiming better engine performance after use.  Personally i am
suspicious of fuel additives.  The oil refiners who produce these fuels
are the experts in the area of additives.  If the supplemental fuel
additives were really needed, then they would already be present in the
fuel at the pump.  Finally, due to the lower sulphur level in current
Canadian (and US) diesel fuels, a number of injection pump failures have
been reported.  Apparently the rotating vane pump designs are more
sensitive to lower fuel lubricity.  The older piston style pumps (as
found on 300SD and 300D Turbo engines) do not seem as sensitive.
Despite this, i add about one cup of ATF per tank refill to make sure
that the petro diesel is slippery enough for the pump to keep working
properly.  This recommendation comes from my Mercedes Specialists in
town.  Lack of lubricity in the fuel is _not_ an issue when you use
biodiesel or some blend of neat vegetable oil.

And my '87 300SDL has a 25 gallon tank. Gulp.